THE SHORES OF BRITAIN. 65 



nails) is a native of the Mediterranean, but is much 

 more familiar to us than our native species, of which 

 there are many. The appearance which it presents 

 is that of an irregularly-shaped mass, more or less 

 rounded, composed of a brown woolly substance, 

 perforated by innumerable pores in all directions, 

 and having in addition, wide canals communicating 

 with each other, and terminating in round holes or 

 mouths on the surface. But if we take a small por- 

 tion of the substance, and place it under a common 

 magnifying lens, we shall see that it is composed of 

 shining, horny, nearly-transparent fibres, which, by 

 uniting with each other at all angles and distances, 

 form a loose and very irregular network. Now, 

 when in a living state, every fibre was enclosed in a 

 coating of thin, clear jelly, which formed the living 

 animal, the horny fibres constituting, as I have inti- 

 mated above, only the skeleton. Imbedded in the 

 substance of many species, some British ones, for 

 example, are found spicule?, or needle-like crystals, 

 of pure flint, varying much in shape in various kinds, 

 while other species have similar crystals of lime. 

 Where these occur in considerable numbers, the 

 Sponge does not possess elasticity : it may be 

 crushed, but it will not reuain its original form. 

 It is a singular fact, that Sponges of these three 

 different kinds are sometimes found growing close 

 to each other, and all alike nourished by the same 

 simple fluid, pure sea- water ; yet they elaborate 

 therefrom products so different as horn, flint, and 

 lime. The animal nature of Sponges is not easily 

 to be detected : no indication of sensation has ever 



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